This invention relates to charged particle storage rings and radiation sources, specifically to sources using radiation from the perturbation of relativistic charged particles. The invention provides a way to increase the total electron or charged particle flux available for use with radiating targets in a storage ring.
It has been considered by many that it is impossible to inject an electron into a magnetic storage ring from an external location without the use of time-varying, inhomogenous magnetic fields or synchrotron radiation. (D. W. Kerst and R. Server, Phys. Rev., vol. 60, pp.53-58, 1941.) An electron or other charged particle launched into a static magnetic field from a point exterior to that magnetic field and which experiences no acceleration other than that provided by the static magnetic field cannot subscribe to a path completely contained within that field. Charged particles are trapped into magnetic storage rings by either modifying the magnetic field while the particle is in the storage ring, by such means as a xe2x80x9ckickerxe2x80x9d magnet or perturbator, or by modifying the energy or trajectory of the charged particle. For instance, the synchrotron radiation emitted by high energy electrons in a large magnetic storage ring slows the particle, increasing the effective force of the magnetic field, and giving the magnetic storage ring the turning power needed to capture the electron. Kicker magnets and perturbators are used to modify the magnetic field to capture the electrons.
Another method used to capture electrons or other charged particles in a storage ring is to inject the particles from a point inside the ring. This method is used in betatrons.
Electromagnets capable of changing the strength of their magnetic field quickly, often called xe2x80x9ckickerxe2x80x9d magnets are used to capture externally injected charged particles in a magnetic ring such as a synchrotron. For a magnetic ring of 10 meters diameter, the travel time around the ring is 100 nanoseconds. In addition, betatron oscillations will prevent the electron from returning close to its point of injection for several cycles, allowing the kicker magnet a time on the order of microseconds to switch. For small magnetic storage rings, of diameter 1 meter or less, the travel time for one orbit around the ring is on the order of 10 nanoseconds, which is makes kicker magnets prohibitively difficult and expensive to produce. The difficulty increases with decreasing radius. Examples a such systems can be found in U.S. Pat. Nos. 5,789,875, 5,216,377 and 5,001,437.
Perturbators are generally air-core coils that generate a non-linear magnetic field in the radius vector direction (see U.S. Pat. No. 5,680,018). They are similar to kicker magnets, but use a weaker field, allowing their use with smaller storage rings. Using a method called resonance injection, the perturbator is driven for a period on the order of 100 nanoseconds, allowing electron capture during this time. The perturbing field increases the betatron oscillations of injected particles, keeping them away from the point of injection for multiple orbits. When the perturbator is turned off, the beam size decreases and the betatron oscillations decrease due to radiation damping, with particles settling on a center, equilibrium orbit. The perturbator can be constructed to minimize disturbance to particles already at the equilibrium orbit. Thus, the perturbator can be pulsed again, allowing further injection, only after sufficient radiation damping to move the already injected particles away from the perturbing magnetic field. Typically this allows injection pulses at a repetition rate of 100 Hz, for a duty cycle of 10xe2x88x929. This method does not allow for truly continuous injection (a 100% duty cycle) and requires, like a kicker magnet, a complex, rapid magnet pulse system.
The energy lost by an electron or other charged particle as it accelerates (turns) in the magnetic field can be used to slow the particle and allow its capture. This is in part used for resonant injection with a perturbator. However, the energy emitted by a charged particle as synchrotron radiation varies as the fourth power of the electron energy. More precisely, for electrons the energy loss due to synchrotron radiation is (D. H. Tombaoulion and P. L. Hartman, Phys. Rev. vol. 102, pp. 1423-46, 1956.):       Δ    ⁢          xe2x80x83        ⁢          E      ⁡              (                  K          ⁢                      xe2x80x83                    ⁢          e          ⁢                      xe2x80x83                    ⁢          V                )              =      88.5    ·                            (                                    E              e                        ⁡                          (                              G                ⁢                                  xe2x80x83                                ⁢                e                ⁢                                  xe2x80x83                                ⁢                V                            )                                )                4                    R        ⁡                  (          meters          )                    
where xcex94E(KeV) is the energy loss of the electron expressed in kiloelectron volts, Ee(GeV) is the initial energy of the electron in gigaelectron volts and R(meters) is the radius of the magnetic storage ring in meters. For a 1 GeV electron in a 1 meter diameter ring, the energy loss would by 88.5 KeV or a relative change of 88.5xc3x9710xe2x88x926. Although a change in energy on the order of 10xe2x88x924 is small, it can be sufficient to trap an electron in a magnetic field if its initial trajectory is close to that of a closed path within the magnetic ring. However, a 100 MeV electron would experience an energy loss of 8.85 eV or approximately on part in 107. This results in an unacceptably small deviation of the injection path from a closed path in the magnetic ring. For 10 MeV electrons, even using a ring of 10 cm diameter, the energy loss is 8.85 meV or approximately one part in 109. Thus, synchrotron radiation is a highly inefficient braking mechanism for the capture of 100 MeV or smaller energy electrons in a magnetic storage ring. Nakayama (U.S. Pat. No. 4,988,950) describes some of the difficulties of injecting a low energy electron beam. These include that the lifetime of a low energy (40 MeV) beam is typically several minutes, which is sufficient for use with a solid radiator target, but is not compatible with the long storage times needed to build up a the large current necessary for intense synchrotron sources. Note that Nakayama uses a pulsed deflection magnet for electron beam capture.
Electron storage rings have been proposed using a gas, such as hydrogen, to focus electrons injected into a storage ring. In addition, a thin solid target is proposed as a means of enhancing the radiation production of this device. It has been recognized that the gas and thin solid target act to dampen the electron beam, and that this damping can increase the repetition rate for resonance injection. However, resonance injection, using a perturbator with a magnetic field that turns on and off in approximately 0.1 microseconds is still required. The use of a perturbator greatly increases the cost and complexity of the system. H. Yamada, U.S. Pat. No. 5,680,018.
One means to capture such lower energy electrons in a storage ring is to inject them from a point inside the magnetic ring. An example of this is a betatron, where electrons are injected from a point inside the radial containment field. (D. W. Kerst, Phys. Rev. vol. 60, pp.47-53, 1941; R. Kollath Particle Accelerators (Pitman and Sons: London) 1967. However, the requirement to inject from an internal point limits the size of the injector and thus the maximum energy of injection. The typical injection energy for betatrons is around 50 KeV. The electrons are then accelerated to higher energies by magnetic induction in the betatron. However, the efficiency of injection into the magnetic ring at energies below 1 MeV is limited by space charge effects, limiting betatrons to average of 10 xcexcA or less, much smaller than the current available from linear accelerators, which can be on the order of mA""s. Indeed, typical betatron currents are 1 xcexcA or less. In addition, the injection into a betatron must be timed with the accelerating field, and a large time-varying magnetic field must be provided for acceleration. These requirements limit the operating frequency of a betatron to approximately 1 KHz or less. Continuous injection is not possible.
The invention uses a solid target in a magnetic storage ring to slow, and capture in a magnetic field, particles injected from a point external to the magnetic field. No magnetic pulsing system is required, and electron energies from several KeV to GeV can be captured. The braking target can also be used to produce radiation. Since the particle beam is in a storage ring, it can pass multiple times through the target, providing much greater efficiency than a single pass radiator system. As an example, a 30 MeV electron directed into a magnetic ring and passing through a 34 micron beryllium foil within this ring would lose 10 KeV of energy or 0.03%. This loss effectively increases the strength of the magnetic field by 0.03%, thus creating the same effect as a kicker magnet but in the 113 femptoseconds that it takes the electron to traverse the foil. The effective increase in magnetic acceleration allows the field of the magnetic ring to hold the electron in a closed orbit. Since only a small fraction of the electron""s energy is lost on each pass through the target, it can potentially pass through the target hundreds of times.
External injection permits many more electrons (or other charged particles) to be captured in the storage ring than is possible using internal injection, greatly increasing the intensity of radiation from the radiator target over that offered by betatrons or other internal injection methods.
The current method of external injection is passive, allowing for continual injection, rather than a limited duty cycle of pulses as for methods based on varying magnetic fields.
The current method of external injection is passive, eliminating the need for a high-speed pulsed magnet, thus reducing system cost and size and increasing reliability.
The current apparatus and method can use storage rings much smaller than one meter in diameter since particles are captured within femptoseconds. In principle, the only limit on size is the ability to construct a magnet and radiation target of a given size.
The current apparatus and method can capture electrons with energies below 100 MeV, which is very difficult using traditional methods based on synchrotron radiation, both because the synchrotron damping is insufficient and because long electron beam lifetimes (many minutes) are required to convert the electron""s energy to synchrotron radiation. Lower energy electron beams are inherently less stable; however, the present invention extracts the electron""s energy into radiation in a fraction of a second.
The charged particle beam can pass through the radiator multiple times, greatly increasing the radiation efficiency over that from a single pass from the injector, such as could be achieved using a linear accelerator and radiation target without the storage ring.
Much lower electron beam energies can be used. Solid targets are more efficient x-ray generators per electron than synchrotron radiation, especially at low electron beam energies. This greatly decreased the size and cost of the apparatus for a given radiation energy over that for synchrotrons or other storage ring radiation sources.
Advanced methods of radiation generation can also be used, including transition radiation, parametric radiation, Cerenkov, bremsstrahlung, coherent bremsstrahlung.
Thin braking targets can be combined with thick bremsstrahlung radiating targets to produce an intense microspot bremsstrahlung source with radiation source dimensions of 100 microns square or even smaller.
Vacuum requirements are much lower than for synchrotrons or other storage rings, since the charged particles need only pass through the target hundreds or thousands of times to convert their energy to radiation. A vacuum of 106 Torr is required rather than 10xe2x88x9210 Torr as for most storage rings.
The energy of the resulting radiation can be controlled by the type of radiator target used, so that the radiation energy can be chosen independent of the electron energy.